It is known to investigate sample systems with electromagnetic radiation by application of ellipsometers, polarimeters, reflectometers, spectrophotometers and the like. Prior art describes the use of lenses to focus a beam of electromagnetic radiation onto a sample and to recollimate it thereafter, and known Patent Applications describe use of focusing and/or collimating “Achromatic” Lenses before and/or after a sample. Ideally an “Achromatic” lens provides the same focal length at all wavelengths in a beam of electromagnetism, however, practical “Achromatic” Lenses have focal lengths which vary with wavelength, in a cyclic manner about an average. That is, a plot of Focal Length vs. Wavelength rises and falls such that a line drawn substantially parallel to the Wavelength Axis passes through said plot a plurality of times. At said Focal Length then at least two, and typically more, wavelengths for which the Focal length is the same are identified. Achromatic Lenses can be designed to set two desired wavelengths, (eg. 193 nm and 248 nm), and often others will also result, on a determined, non-selected basis, at which the Focal Lengths are equal.
In ellipsometry it is often desirable to take data which pertains to at least two selected wavelengths, and it is also important to investigate a sample with said multiple wavelengths at exactly the same spot thereupon as well as detect resulting data for each wavelength similarly. This requires equal source and detector side focal lengths at said wavelengths.
The disclosed invention then is a system of lenses which are designed to have equal focal lengths at a plurality of wavelengths, and a method of their use in analyzing a specific spot on a sample using electromagnetic radiation applied at an oblique angle.
Another aspect of the disclosed invention is that a plurality of lens sets can be designed which provide different combinations of wavelengths at which focal lengths are equal, and said plurality of sets of lenses can be mounted in, for instance an ellipsometer system, to allow them to be sequentially positioned in the path of an electromagnetic beam, so that more wavelengths can be sequentially caused to focus on the same spot on a sample.
In another variation, a single lens can be mounted so that it can be moved toward and away from a sample, so as to sequentially cause different wavelengths to focus on the same spot on a sample at different wavelengths.
It is also noted that supplementing a minimal number of wavelengths with increased number of angles-of-incidence can provide sufficient data sets where necessary.
A search of Patents provide:
Patent to Ebstein, U.S. Pat. No. 5,091,801 which describes a nearly index matched optic formed of at least two elements for adjusting focal lengths;
a Patent to Noyes, U.S. Pat. No. 5,864,436 which describes an objective lens which has the same focal length at two wavelengths;
a Patent, U.S. Pat. No. 5,973,846 to McConica which describes an auto focus system for a digital camera which has the focus of two spectra offset from one another; and
a Patent, U.S. Pat. No. 5,078,513 to Spaulding et al., which describes a lens in a waveguide of an integrated optical waveguide which is corrected for chromatic dispersion.
Even in view of the prior art, a need remains for improved ellipsometer, polarimeter, reflectometer, spectrophotometer and the like systems which include lenses that provide the same focal length at least two wavelengths.